Three patients with bacteremia caused by unusual gram negative bacteria came to our attention. Two were Brutons agammaglobulinemia patients (NIAID), the third an HIV patient whose organism was referred to us for identification (VAMC). The three organisms shared features of Helicobacter-like organisms based on growth characteristics and gram stain morphology, but biochemically were not compatible with described species. We initiated multiple studies to fully characterize these organisms, including rRNA gene sequencing. The organisms were extremely fastidious, microaerobic curved, and helical bacteria that required specialized conditions to obtain satisfactory growth. Extensive biochemical studies as well as electron microscopy were performed to provide full descriptions of each isolate. Both Brutons patients had organisms that by rRNA gene sequencing aligned most closely with an unusual organism, Flexispira (Helicobacter) rappini. However, DNA hybridization studies (CDC) showed that at least one of the isolates was a new taxon, distinct from F. rappini as well as from other known Helicobacter sp. It is likely that the second F. rappini-like organism is similar, but not identical to the first, based on a comparison of their rRNA sequences. The third organism, thought to be a Campylobacter by the referring hospital, did not fit known species of Campylobacter or Heli-cobacter. Sequencing of the rRNA gene of this organism showed it to be most closely related to a recently described Helicobacter thus far described from only two cases in Australia. These cases help to establish that fastidious species of Helicobacter (non-H. pylori) can be significant pathogens in the immunocompromised host, causing septicemia with notable recurrences if not detected and treated appropriately. The species that cause sepsis are a closely related group by rRNA sequencing and cannot be accurately identified by sequencing alone. We have shown that organisms with greater than 99% base pair identity in the 16 S rRNA gene may show less than 70% homology by total genomic DNA hybridization. This is indicative of a species level difference. The difficulty in detection as well as identification of these organisms is a problem for diagnostic microbiology laboratories. Special culture methods must be used to grow the organ-isms, then molecular methods must be used for definitive identification.